114 



CARNEGIE INSTITUTION OF WASHINGTON. 



plainly marked by the attachment of the nerve-roots, for these become 

 securely attached to the cord before the displacement begins and thus 

 permanently mark the various segmental levels. In the case of each 

 segment of the spinal cord there are two fixed topographical points: 

 (1) the spinal ganglion, which is held in the intervertebral foramen and 

 registers the original position of the segment relative to the vertebral 

 column; and (2) the place at which the dorsal root is attached to the 

 cord and which moves as the cord moves. By locating these points 

 for the different stages, one can determine the exact elongation of the 

 nerve-roots, and this in turn is the index of the relative displacement 

 of the spinal cord as regards the vertebral column. Conversely, it will 

 be seen that the alteration not explained by mechanical displacement 

 must be attributed to the retrogressive changes referred to. 



22l.mm (xi») 



Topographical relations of caudal end of spinal cord in the human fetus from the eighth to the 

 twenty-fifth week, showing formation of the filum terminale. By comparing these stages, one 

 can determine the rate and extent of caudal displacement of the vertebral column relative to the 

 terminal ventricle and the attachment of the sacral nerve-roots, which constitute definite and 

 fixed points on the spinal cord. In each case the dorsal root of the first sacral nerve is drawn in, 

 and the point of attachment of the dorsal roots of the other sacral ner\^es is shown by straight 

 lines. The twenty-fifth to the twenty-ninth vertebrae were regarded as sacral vertebra and are 

 shown in stipple. The fetuses are listed in the Carnegie Collection as follows: No. 75, 30 mm.; 

 No. 1656, 67 mm.; No. 1673, 111 mm.; No. 1.395, 221 mm. 



In this way I was able to determine that in the human embryo the 

 greater part of the coccygeal and post-coccygeal cord — that is, the 

 part caudal to the thirtieth segment — undergoes dedifferentiation, the 

 more cephalic part of it persisting as the ventriculus terminalis and the 

 more caudal part redifferentiating into a fibrous strand — the filum 

 terminale — with the coccygeal medullary vestige at the tip. The first 

 29 segments of the spinal cord are not affected by this process of 



